Facebook, the world’s most popular social-networking service, was sued over claims it secretly tracks its users’ Web activity after they log off.

The company assures users that “cookie” files installed on their computers to identify them and track their interactions with Facebook applications and Web sites while they are logged on are removed when they log off, according to a complaint in federal court in San Jose, Calif. Facebook admitted on Sept. 26 that the cookies track users’ Internet activity after they log off, according to the complaint.

“This admission came only after an Australian technology blogger exposed Facebook’s practice of monitoring members who have logged out, although he brought the problems to the defendant’s attention a year ago,” according to the complaint.

On Thursday, 10 public-interest groups asked the US Federal Trade Commission to investigate Facebook’s tracking of Internet users after they log off. The Electronic Privacy Information Center and nine other groups urged the FTC to examine whether Facebook’s new Ticker and Timeline features increase privacy risks for users by combining biographical information in an easily accessible format.

The lawsuit filed by Perrin Aikens Davis seeks class, or group, status on behalf of other Facebook users living in the US. Davis requested unspecified damages and a court order blocking the tracking based on alleged violations of federal laws, including restrictions on wiretapping, as well as computer fraud and abuse statutes.

“We believe this complaint is without merit and we will fight it vigorously,” Facebook said.